Gas filtering and de-misting devices have been used for many years. There are roughly three fundamental types of apparatus used to filter gas; that is, to remove solid impurities from a gas stream. The same apparatus can be used to de-mist gas; that is, remove (or lower) the entrained liquids found in gas produced and used within the process industries (which includes natural gas production, oil production--with produced-gas, and refining).
Basically the dirty wet gas is passed through a sealed pressure vessel called a filter-separator which can be a horizontal or a vertical vessel depending on site requirements. If the gas is perfectly dry, there is no need for separation, only filtration. But, normally, both steps are performed within a single filter-separator. Traditionally, separation of entrained liquids from the gas is performed in one of three ways:
a wire mesh impingement device within the vessel, PA1 a centrifugal impingement device within the vessel, or PA1 a vane type impingement device within the vessel.
With the advent of newer materials, there is now a fourth method which combines the filter and an impingement device to allow the entrained liquids to coalesce within the filter and along the filter boundaries. This process is called "reverse flow filtering and mist coalescence". The term "reverse flow" comes from the fact that the gas flows backward through the filter rather then forward through the filter as used in the standard filter-separator. This terminology is somewhat important because standard filters are supported by an internal structure and in forward flow the filter is pressed against that support structure. In reverse flow, the filter is pulled away from the support structure and the filter must be capable of carrying the imposed mechanical load. That load would be related to force (differential pressure drop across the filter) and area. The elements in a filter-coalescer serve two purposes, the first being a filter and the second being the area over which the entrained liquid can coalesce. If the rate of flow of the gas over the element is kept within a reasonable low flow, then the liquid droplets which form within the element and along the boundaries of the element will not be picked up by the gas and carried out of the vessel. These droplets are allowed to fall (under gravity) to the bottom of the vessel and are removed using standard liquid level control processes.
Reverse flow filter-coalescers appeared in the industry in the 1960's and standard filters and filter-separators came of age after the Second World War, although the concepts had been practiced long before by industry. One of the problems associated with both types of elements, used in these devices, is found in their filtration function. After a period of time, the filter traps so much material that the filter is no longer efficient and it stops working. The element must be replaced. Traditionally, this meant that the filter separator had to be taken out of service: an operation that can be intolerable to a chemical process. It is possible to have two such vessels, one in service and one in standby. However, if large amounts of gas need to be filtered or de-misted, the pressure vessel is subsequently large and the cost for two vessels can be prohibitively high. Thus, two such vessels are rarely found in the industry.